The composition is pretty balanced and the characters proportions both alone and in relation to each other is great. That's a pretty nice hand there too. •́ε•̀ The characters really have a lot of personality and that's often hard to capture. I think you did that very well.

You added a lot of detail to the background, but the perspective needs some work. It might be helpful to look up some tutorials on how to use a vanishing point to achieve perspective. The cool thing about a background like this is that it's a lot of straight edges so you can use the pen or line tool to it's full advantage here. I wouldn't have known you used the auto-color feature for the background either. I haven't had much luck getting it to work myself. XD It's a weird feature I haven't figured out yet, so good job! It worked pretty well! =D

Let's talk about colors! The colors you used for the characters are SUPER dark compared to the background and it makes it a little hard to see the details in the shading as well since there isn't much contrast. One suggestion I have is to avoid using black whenever possible. Even when the character has a "black" shirt, make it a dark grey and even maybe add some color in there too. Like a really desaturated purple or something. You'll find adding a bit of color like that can help with shading. Speaking of shading, it may aslo be helpful to be mindful of where your lightsource is. Even if the room is fully lit, things are going to cast shadows (like hair, clothes, etc.) so think about where those are going to land and it will help give objects a little more depth.

Overall I think you did a great job and I'm looking forward to your next drawing. =) Rest your hands first though.